During the duty cycle of a machine the conditions at the outlet of a cylinder type fluid motor may vary widely and the very undesirable condition of cavitation may take place. As is well known to those skilled in the art, cavitation may adversely affect the life of the system components, especially the system pump, generate noise and introduce very undesirable characteristics when controlling a load. For example, when controlling a negative load, acting in the direction of the piston rod of the cylinder, the inlet flow to the cylinder has to be supplemented by flow, equivalent to the displacement of the piston rod. Such an inlet flow may have to be supplemented, when the velocity of the negative load creates an inlet flow requirement higher than the capacity of the system pump, or when the flow from the system pump is not available, which is the case either during so-called "negative load regeneration", or when the flow at negative load is not diverted to the exhaust manifold. A typical negative load regeneration system is shown in U.S. Pat. No. 4,267,860 which issued May 19, 1981 to Tadeusz Budzich.
The condition of negative load regeneration, in the embodiment described in this specification, takes place when the controlled load is of an aiding or negative load type and when the flow of fluid from the system pump is isolated from the fluid motor. The control of the position of the load is accomplished by using the potential energy of the load. Under these conditions the flow from the pump can be directed to perform useful work in the control of other resistive, or positive type loads of the system. Under these conditions additional flow can be supplied to the exhaust manifold either from an additional low pressure pump, which in a mobile type circuit may be undesirable, since it requires a separate power take-off, or from a fluid flow amplifying device, using energy derived either from the negative load, or from the system pump. Since under the above conditions some of the flow transfer may have to take place through the anticavitational controls of a mobile type valve, it is to a great advantage to supply the necessary make-up flow at a preselected pressure level, higher than atmospheric. Also by maintaining both ends of a cylinder above a certain minimum pressure level, by compressing the entrained air, which is always present in the circulating oil, the system stiffness is substantially increased, which in turn produces a number of very beneficial effects. It is also very desirable, when controlling a positive load in mobile type circuits, to completely unload the pressure of the exhaust manifold, not only in order to increase the system efficiency, but also to increase the level of the effective force, developed by the cylinder, especially when raising a load using the maximum output flow from the system pump.